Control valves are commonly used in process control systems. In general, a control valve may be used to manipulate a flowing fluid to regulate a processes variable to a desired set point, to convey or deliver a fluid from a source to a destination, etc. A control valve assembly typically includes a valve body, a shaft, and an actuator to provide the motive power via the shaft to operate the valve (e.g., to position a plug or flow control member within the valve).
Spring and diaphragm pneumatic actuators are commonly referred to as diaphragm actuators and are often selected for use with control valves due to their dependability. Diaphragm actuators typically include a casing containing a diaphragm, an actuator shaft, and one or more springs to return the actuator shaft to a known position in the absence of a control pressure applied to the diaphragm. Typically, diaphragm actuators receive a variable gas (e.g., air) pressure on one side of the diaphragm to move the actuator shaft and thereby open and close or modulate a control valve. The control action of the actuator depends on the configuration of the diaphragm, the diaphragm plate, the return springs, and the actuator shaft. Diaphragm actuators can be configured to be direct acting so that an increased control pressure extends the diaphragm and actuator shaft away from the actuator casing. Alternatively, diaphragm actuators can be configured to be reverse acting so that increased air pressure retracts the diaphragm and the actuator shaft.
Regardless of the type of control action for which a diaphragm actuator is configured, the actuator shaft, which may also be referred to as a stem, is mechanically coupled or fastened to the diaphragm. Typically, an end portion of the shaft or stem passes through a central aperture of the diaphragm, and one or more mechanical fastening elements (e.g., washers, nuts, bolts, etc.) are used to clamp or otherwise fix the central portion of the diaphragm to the shaft or stem. In this manner, movements or displacements of the diaphragm in response to control pressure changes cause corresponding movements or displacements of the actuator shaft or stem. To enable a control pressure to be maintained on one side of the diaphragm, the aperture through which the end of the actuator shaft or stem passes is typically sealed to prevent leakage around the shaft or stem through the aperture. However, known techniques for sealing the central aperture of a diaphragm through which a shaft or stem passes typically use a relatively complex and expensive arrangement of washers, o-rings, and specialized components.